Wafer transfer device

ABSTRACT

A wafer transfer device is intended to transfer wafers between a wafer boat in which a plurality of wafer support plates are vertically arranged at regular intervals and a cassette in which a plurality of wafer-mounted levels are vertically arranged at regular intervals. Each of the wafer support plates is ring-shaped having an opening in the center thereof and a passage is defined by openings of the wafer support plates, extending vertically in the boat. The wafer is horizontally transferred into and out of the wafer boat between the wafer support plates by a fork. The fork can be moved in vertical and horizontal directions and it can also be swung. A wafer push-up disk is arranged movable up and down through the passage in the boat. Three wafer supports are projected from the top of the push-up disk. These projections on the push-up disk are arranged contactable with the underside of the wafer without interfering with the fork. The fork and the push-up disk are associated with each other by a controller to achieve the transferring of wafers.

This is a division, of application Ser. No. 07/907,545, filed on Jul. 2,1992, now U.S. Pat. No. 5,275,521.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer transfer device and moreparticularly, a device for transferring wafers between wafer boat andcassette.

2. Description of the Related Art

A wafer housing case which is called carrier or cassette is usually usedto transfer semiconductor wafers and others in the course ofmanufacturing semiconductor devices.

The wafer cassette is made of resin, for example, light in weight andlow in cost, and it is designed to house plural or 25 sheets of waferstherein.

When a plurality of wafers are to be processed as a batch by the heatprocess system, they cannot be heat-processed under the condition thatthey are kept housed in the resin-made cassette. It is therefore usuallyneeded that they are heat-processed after they are transferred into theheat processing wafer boat made of quartz, for example, chemicallystable and excellent in heat resistance.

As shown in FIGS. 1 and 2, a plurality wafer-mounted levels arevertically arranged in a wafer boat 10 so as to process a plurality ofwafers as a batch. A ring-shaded shaped support plate 11 having arecessed support face 11a thereon (see FIG. 2) has been used these daysas the wafer-mounted level, because the circumferential portion of thewafer can be surrounded by the ring-shaped support plate 11. When thewafer is supported in this manner, its temperature can be uniformlylowered and raised so as to form a film of uniform thickness on thewafer.

When the wafer is transferred between the ring-shaped support plate anda transfer fork, however, the up- and down-movement of the fork isdisturbed by the ring-shaped support plate. This makes it necessary tomove the wafer up and down independently of the fork. To meet this need,an inventor of the present invention has provided in U.S. patentapplication Ser. No. 07/572,005 (now U.S. Pat. No. 5,162,047 issued Nov.10, 1992) double fork structure in which two wafer transferring andpushing-up forks are used.

When this double fork structure is employed, however, intervals betweenthe wafer-mounted levels or ring-shaped support plates 11 must be madelarge to insert it.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a wafertransfer device capable of making smaller the intervals betweenring-shaped wafer support plates in the heat processing wafer boat.

According to the present invention, there can be provided a wafertransfer device for transferring wafers into and out of a wafer boat inwhich a plurality of wafer support plates are vertically arranged havingintervals between them, the ring-shaped wafer support plates beingring-shaped and defining a vertically-extending passage by centeropenings thereof, said wafer transfer device comprising: carrying meansfor horizontally carrying the wafer into and out of the wafer boatbetween the wafer support plates; a wafer push-up disk arranged movableup and down through the passage; wafer support means projected from thetop of the push-up disk and arranged contactable with the underside ofthe wafer without interfering with the carrying means; drive means fordriving the push-up disk up and down; and means for controlling thecarrying means and the push-up disk such that they can be associatedwith each other.

According to the present invention, the underside of the wafer isreleased from the ring-shaped wafer support plate in the wafer boat bythe push-up mechanism. This enables a gap, into which the carrying meansis inserted, to be provided between the under-side of the wafer and thesupport plate. Further, the push-up mechanism is arranged movable up anddown through the passage defined by center openings of the supportplates. This makes it unnecessary to insert the wafer-lifting means intothe wafer boat between the support plates. The intervals between thesupport plates in the boat can be thus made smaller.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a sectional view showing a wafer boat loaded in apressure-reduced CVD system of the vertical type which is an example ofa heat process system;

FIG. 2 is a perspective view showing a part of the wafer boat enlarged;

FIG. 3 is a plan view showing how each member of the heat process systemis positioned with reference to a position just under the heatingfurnace and another position where the wafer transfer device of thepresent invention is applied;

FIG. 4 is a perspective view showing the wafer transfer device accordingto a first embodiment of the present invention;

FIG. 5 shows the wafer boat mounted on a stand when viewed from the top:

FIG. 6 is a sectional view taken along a line VI--O--VI in FIG. 5;

FIG. 7 is a perspective view showing in detail a drive section for awafer push-up mechanism of the wafer transfer device according to thefirst embodiment;

FIGS. 8A and 8B show how the wafer push-up mechanism of FIG. 7 isoperated;

FIG. 9 is a perspective view showing a wafer transfer fork;

FIG. 10 is a perspective view showing an example of the manner of makingthe fork;

FIG. 11 is a perspective view showing another example of the manner ofmaking the fork;

FIGS. 12 through 14 are sectional views taken along a line XII--XII inFIG. 5 and showing how the wafer is transferred into the wafer boat;

FIG. 15 is a sectional view showing how the fork is related to a push-updisk;

FIG. 16 is a perspective view showing the conventional fork which isused as a comparison example;

FIG. 17 is a perspective view showing the wafer transfer deviceaccording to a second embodiment of the present invention;

FIG. 18 is a perspective view showing in detail a drive section for awafer push-up mechanism of the wafer transfer device according to thesecond embodiment; and

FIGS. 19A through 19C schematically showing how the wafer push-upmechanism of FIG. 18 is operated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view showing a wafer boat 10 loaded in thepressure-reduced CVD system of a vertical type which is an example ofthe heat process system. A heating furnace 1 includes an outer shellmade of a metal plate 2a and a heat insulating layer 2b, and a heater 2carranged along the inner wall of the outer shell. A cylindrical reactortube 3 is erected in the heating furnace 2. A manifold 4 made ofstainless steel, for example, is connected to the lower end of thereactor tube 3. Gas supply pipes 8a and 8b through which film forminggases are introduced into the reactor tube 3 are connected to themanifold 4. An exhaust pipe 9 through which gases are exhausted outsideafter the reaction is also connected to the manifold 4.

The wafer boat 10 is supported on a heat insulating sleeve 6 such thatits lower end portion is inserted into the sleeve 6. The heat insulatingsleeve 6 is on a turntable 6a. A rotating shaft 6b of the turntable 6ais connected to a drive mechanism in a support 7a of a lifter unit 7. Acap 5 is attached to a base 6c of the rotating shaft 6b. when the lifterunit 7 is moved up and down, the open bottom of the manifold 4 is closedand opened by the cap 5.

FIG. 2 is a perspective view showing a part of the wafer boat 10enlarged. The wafer boat 10 comprises four columns 12 and plural or 60sheets of ring-shaped wafer support plates 11. Each of the supportplates 11 has a wafer support face 11a which is so recessed as to keepthe top surface of the wafer aligned with that of the wafer supportplate 11 when the wafer is supported on the wafer support plate 11.

The wafer boat 10 which is intended to hold the 6-inch wafers thereinhas the following dimensions:

The outer diameter of each ring-shaped wafer support plate: φ156 mm,

the inner diameter of each ring-shaped wafer support plate: φ123 mm,

the outer diameter of the wafer support face: φ153 mm,

the thickness of each ring-shaped wafer support plate: 3 mm (however,that of the wafer support face: 2 mm), and

the intervals between the adjacent ring-shaped wafer support plates:9.525 mm.

FIG. 3 is a plan view showing how each member of the heat process systemis arranged with reference to a position P1 just under the heatingfurnace and another position P2 where the wafer transfer device of thepresent invention is applied. After the wafers which have been held inthe wafer boat 10 as shown in FIG. 1 are heat-processed, the wafer boat10 is lowered to the position P1 by the Lifter unit 7. It is thenreleased from the heat insulating sleeve 6 by a swing arm 1 and carriedfrom the position P1 to the position P2 where it is mounted on the stand15 (see FIG. 4). The wafers which have been processed are transferredfrom the wafer boat 10 to a cassette 50 by the wafer transfer device ofthe present invention which includes a transfer mechanism 30.

On the other hand, the wafers which are not processed yet aretransferred from a cassette 50 to the wafer boat 10 on the stand 15 atthe position P2 by the wafer transfer device of the present invention.The wafer boat 10 is then released from the stand 15 by the swing arm 1and carried from the position P2 to the position P1 where it is mountedon the heat insulating sleeve 6. It is loaded into the reactor tube 3,as shown in FIG. 1, by the lifter unit 7.

FIG. 4 is a perspective view schematically showing the wafer transferdevice according to a first embodiment of the present invention. Thiswafer transfer device of the present invention includes push-up andtransfer mechanisms 20 and 30 which are positioned adjacent to the wafercassette 50. The push-up and transfer mechanisms 20 and 30 arecontrolled and operated by a controller 35 and a computer system 36,associating with each other.

The wafer cassette 50 is mounted on a cassette mount 55. It has pluralor 25 mount levels which are arranged at the same pitch in the verticaldirection and the wafers are placed on these mount levels in it.

The transfer mechanism 30 includes a transfer fork 31, a slide block 32and a rotary base 33. The transfer fork 31 is a plate having a topsurface contacted with the underside of the wafer, and it is arranged onthe slide block 32 so as to slidably reciprocate in a direction X. Thetransfer fork 31 can be swung in a direction θ and further reciprocatedin a direction Z by the rotary base 33.

FIG. 5 shows the wafer boat 10 mounted on the stand 15 when viewed fromthe top, and FIG. 6 is a sectional view taken along a line VI--O--VI inFIG. 5.

Grooves 13 are formed on each of the columns 12 of the wafer boat 10 atregular intervals and each of the ring-shaped wafer support plates 11 issupported by the columns 12 such that it is fitted into one of thegrooves 13 of the columns 12 which are at a same level. Each of thewafers is supported on the support plate 11 with its underside contactedwith the wafer support face 11a.

The wafer push-up mechanism 20 includes a drive section 23 housed In thestand 15, and a push-up disk 21 movable through a space or passage whichis defined by openings of the ring-shaped wafer supports 11 of the boat10. The push-up disk 21 has three projections 22 which are contactedwith the underside of the wafer to support the wafer on them.

FIG. 7 is a perspective view showing a drive section 28 for the waferpush-up mechanism 20 in detail. The drive section 28 comprises a motor28m, a screw rod 28l rotated by the motor 28m. a guide 28g, and a firstlifter plate 28a connected to the wafer push-up disk 21 and providedwith a ball screw mechanism. The motor 28m and the guide 28g are housedin a cylindrical cover 29 (see FIG. 4).

FIGS. 8A and 8B are intended to schematically show how the wafer push-upmechanism 20 of the first wafer transfer device is operated. As shown inFIGS. 8A and 8B, the wafer push-up disk 21 can be moved between itslower limit position (FIG. 8A) where it is waiting in the stand 15 anditems upper limit position (FIG. 8B) where it is pushed up by the drivesection 28.

FIG. 9 is a perspective view showing the wafer transfer fork 31 indetail. This wafer transfer fork 31 is a plate, 210 mm long, 56 mm wideand 1.0 mm thick, made of alumina and intended to transfer 6-inchwafers.

Wafer supports 61-64 having circular tops 61a-64a are projected from thetop surface 60a of a body 60 of the fork 31. When the wafer is to betransferred, the tops 61a-64a of the supports 61-64 are contacted withthe underside of the wafer to support the wafer on them. Each of thetops 61a-64a of the supports 61-64 has a diameter of 10.5 mm and it alsohas a height h₁ of 0.3 mm when measured from the top 60a of the forkbody 60. It is not necessarily shaped like a circle but it may be shapedlike a rectangle.

The wafer supports 61-64 are formed as follows: As shown in FIG. 10,recesses U₁ -U₄ are formed on the top 60a of a fork body material 60x.Column-shaped wafer support material 61x-64x are press-fitted into therecesses U₁ -U₄ and CVD coating process is then applied to these wafersupport materials 61x-64x including their tops 61ax-64ax. Or, as shownin FIG. 11, wafer support materials 61y-64y having those tops 61ay-64ayto which the CVD coating process has been applied may be bonded to thetop of a fork body material 60x by adhesive.

In the case of the fork 31, the CVD coating process of SiC is applied tothe wafer supports 61-64 including their tops 61a-64a. A CVD coat layerwhich is composed of fine crystals of SiC is therefore formed on thewhole surface of each of the wafer supports 61-64, thereby making theirsurfaces smooth like a mirror. The CVD coat layer of SiC has so highhardness that it cannot be broken by its contact with the wafer whiletransferring the wafer. In addition, it has extremely high purity andthe amount of impurities such as heavy metal contained in it is smallerthan that in quartz of which the wafer boat is made. Further, it is moreadvantageous particularly in that it contains extremely little ofnatrium which adds bad influence to the semiconductor wafer. Thethickness of the CVD coat layer is about 50 μm, for example.

Reference numeral 65 in FIG. 9 denotes a cut-away portion provided atone end of the fork 31 and another reference numeral 66 representsmembers for preventing the wafer from being shifted from its position onthe fork 31. These members are column-like projections located adjacentto their corresponding wafer supports 61-64 and having a diameter of 5mm and a height h₂ of 0.7 mm when measured from the top 60a of the forkbody 60. The wafer is supported at its rim portion by these stoppermembers 66 not to shift from its position on the fork 31 while beingtransferred.

According to the fork 31 of the first wafer transfer device, the wafersupports 61-64 including their tops 61a-64a which are contacted with theunderside of the wafer are coated with SiC according to the CVD manner.Therefore, their surfaces can be made like a mirror, having highsmoothness. In addition, their areas with which the underside of thewafer is contacted are quite small in total. This can reduce thecreation of dust not to contaminate the surface of the wafer which is onthe way of its being transferred.

The fork 31 may be variously modified. For example, its whole surfacemay be coated like a mirror according to the CVD coating process.Further, it is preferable that the CVD coat layer contains Si from theviewpoint of preventing the wafers from being contaminated. Therefore,SiO₂, SiN or others may be used as well.

An example of the process of transferring the wafers from the wafercassette 50 to the boat 10 will be described with reference to FIG. 4and FIGS. 12 through 15. FIGS. 12 through 14 are sectional views takenalong a line XII--XII in FIG. 5 and some of components are omitted forthe sake of simplicity in them.

1. The slide block 32 is set to oppose the fork 31 to the cassette 50.The fork 31 is then moved in the direction X or toward the cassette 50and positioned under the wafer W.

2. The slide block 32 is lifted to move the fork 31 in the direction Z.The tops 61a-64a of the wafer supports 61-64 on the fork 31 are thuscontacted with the underside of the wafer W to support the wafer W onthem or on the fork 31.

3. The fork 31 is retreated in a direction reverse to the direction X totake the wafer W out of the cassette 50.

4. The slide block 32 is swung in the direction θ to oppose the fork 31to the boat 10.

5. The fork 31 on which the wafer W is supported is inserted between thesupport plates 11 (or 111) and 11 (or 112) of the boat 10, as shown inFIG. 12.

6. The push-up disk 21 of the push-up mechanism 20 is lifted until thewafer W can be held on the projections 22 of the disk 21. The wafer W isthus supported on the tops of the projections 22 while the tops 61a-64aof the wafer supports 61-64 on the fork 31 are separated from theunderside of the wafer W. FIG. 15 shows this state wherein the boat 10is sectioned along a plane parallel to the face of the wafer W. As shownin FIG. 15, the projections 22 on the disk 21 are projected above thefork 31 through the cut-away portion 65 and on both sides of the fork 31without contacting it.

7. The fork 31 is retreated from the boat 10 in the direction reverse tothe direction X. Only the push-up disk 31 is thus left in the opening ofthe ring-shaped support plate 11, as shown in FIG. 13.

8. The push-up disk 21 is lowered. The wafer W is thus supported on thesupport plate 11 (or 112), as shown in FIG. 14.

When the above procedure is repeated relative to every wafer on thesupport plate 11 of the boat, the wafers can be transferred from thecassette 50 to the boat 10.

When the above procedure is reversely repeated, the wafers which havebeen processed can be transferred from the boat 10 to the cassette 50.In short, the wafer on the support plate 11 is pushed up by the push-upmechanism 20 to release the underside of the wafer from the supportplate 11 and the fork 31 is inserted into a gap between the wafer andthe support plate 11 to support the wafer on the tops 61a-64a of thewafer supports 61-64 thereof. The wafer thus supported on the fork 31 isthen taken out of the boat 10.

According to the wafer transfer device of the present invention, theunderside of the wafer is separated from the support plate 11 of theboat 10 by the push-up mechanism 20. The gap into which the fork 31 isinserted can be thus provided between the underside of the wafer and thesupport plate 11.

Further, the push-up mechanism 20 is arranged to move up an down throughthe space or passage S which is defined by the openings of the pluralring-shaped wafer support plates 11. This makes it unnecessary to makelarge the intervals between the support plates 11 of the boat 10. Inother words, the intervals can be kept smaller to thereby enable alarger number of the wafers to be processed every batch process withoutmaking the boat 10 large-sized.

The following test was conducted to study the effect of the CVD coatlayer applied to the fork 31.

The transferring of wafers was carried out by the wafer transfer deviceprovided with the fork 31 and the number of particles stuck to thesurface of each of the wafers thus transferred was measured. Forcomparison, same measurement was made about those wafers which had beentransferred by the wafer transfer device provided with an alumina fork(material: Al₃ O₃ (A479-SS) made by Kyosera Corp.). This alumina forkwas shaped, as shown in FIG. 16, having wafer support faces 91.

The transferring of wafers and the measuring of the number of particlesstuck were carried out in a clean room (the class of cleanness: 10). Thenumber of particles stuck was measured by Surfscan 5500 (made by TencotInstruments). The target of this Surfscan 5500 was those particles whichwere larger than 0.2 μm.

The slide block 32 was reciprocated in directions X and Z without beingswung in the direction θ to thereby transfer the wafers into and out ofthe cassette 50. The stroke and the speed of the slide block 32 drivenin the direction X were 240 mm and 30 m/sec. Those in the direction zwere 4 mm and 4 m/sec.

One cycle comprised exchanging the wafer with a new one, measuring thenumber of particles adhering to the new wafer, transferring the newwafer into and out of the cassette 50, and measuring the number ofparticles stuck to the wafer thus transferred. Average values wereobtained after 500 and 5000 cycles. Results thus obtained were as shownin Table 1.

                  TABLE 1                                                         ______________________________________                                                           Number of Particles                                        Area of Wafer      Stuck                                                      Contacted (mm.sup.2)                                                                             500 Cycles                                                                              5000 Cycles                                      ______________________________________                                        Our Fork                                                                               78.5          15.4      16.8                                         Fork of 602.5          28.4      19.9                                         FIG. 16                                                                       ______________________________________                                    

As shown in Table 1, the number of particles stuck because of dustcreated was smaller in the case of the fork of the present inventionthan in the case of the conventional fork.

FIG. 17 is a perspective view schematically showing the wafer transferdevice according to a second embodiment of the present invention. FIG.18 is a perspective view schematically showing a drive section for apush-up mechanism of this wafer transfer device. Same components asthose of the first wafer transfer device shown in FIG. 4 will be denotedby same reference numerals and description on these components will beomitted.

The second wafer transfer device is different from the first one in thatthe drive section for the push-up mechanism 20 is of 2-stage typecomprising first and second lifter means 23 and 25.

The first lifter means 23 comprises a motor 23m, screw rod 23l rotatedby the motor 23m, a guide 23g, and a first lifter plate 23a connected tothe push-up disk 21 and provided with the ball screw mechanism. Themotor 23m and the guide 23g are housed in a cylindrical cover 24 andfixed to one end of a second lifter plate 25a which will be describedlater. When the screw rod 23l is rotated, the first lifter plate 23a ismoved along the guide 23g to thereby move the wafer push-up disk 21 upand down in relation to the second lifter plate 25a.

The second Lifter means 25 comprises a motor 25m, screw rod 25l rotatedby the motor 25m, a guide 25g, and the second lifter plate 25a havingthe ball screw mechanism, which are housed in a cylindrical cover 26.The cylindrical cover 26 and the stand 15 have slits through which thesecond lifter plate 25a is moved up and down. When the screw rod 25l isrotated, the second lifter plate 25a is moved along the guide 25g tomove the first lifter means 23 up and down.

FIGS. 19A through 19C are intended to show how the push-up mechanism 20of the second wafer transfer device is operated.

FIG. 19A shows the push-up disk 21 waiting in the boat mounted stand 15or kept at its lower limit level. FIG. 19B shows the push-up disk 21lifted to its upper limit level by the first and second lifter means 23an 25. FIG. 19C shows the wafer W pushed up by the push-up mechanism 20.

In FIG. 19B, Z₁ denotes the stroke of the lifter plate 23a in the caseof the first lifter means 23 and Z₂ the stroke of the lifter plate 25ain the case of the second lifter means 25. The sum (Z₁ +Z₂) is equal tothe stroke of the lifter plate 28a driven by the drive section 28 in thewafer transfer device according to the first embodiment and shown inFIGS. 4, 8A and 8B. In short, the push-up mechanism of the second wafertransfer device is driven by the drive section of the 2-stage type.Therefore, the space the second push-up mechanism occupies can be madesmaller. This is a merit in addition to those achieved by the firstwafer transfer device.

When the stroke of the lifter plate driven by the drive section of thepush-up mechanism is long, the wafer transfer device and the heatprocess system of the vertical type provided with this wafer transferdevice are disadvantageous on the basis of a limit in height upon theirbeing transported and carried into a clean room. This may make itimpossible to provide the space the push-up mechanism occupies.Particularly when the wafers are to be transferred into and cut of thehigh wafer boat of the vertical type. It is quite difficult to providethe vertically-extending space the push-up mechanism occupies.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A heat processing system of the vertical type forprocessing wafers, comprising:(a) a process chamber for housing aplurality of wafers at a time; (b) means for heating the wafers housedin the process chamber; (c) a wafer boat supporting the wafers in theprocess chamber with an interval between adjacent two wafers in avertical direction; (d) boat conveying means for loading and unloadingthe boat into and out of the process chamber with the wafers supportedon the boat; (e) an arm apparatus for transferring the wafers into andout of the boat, which is set substantially in a vertical state, at atransfer position outside the process chamber, the arm apparatuscomprising, an arm main body having a wafer handling face, driving meansfor driving the main body in a horizontal plane, a plurality ofsupporting protrusions arranged on the handling face of the main bodyand having wafer supporting top portions, respectively, to support oneof the wafers, the supporting protrusions being formed separately fromand attached to the main body, and a CVD coating layer coating the topportion of each of the supporting protrusions, the coating layer beingmade of a material containing Si therein and having a high hardness suchthat the coating layer does not contaminate the wafer with dustgenerated therefrom and impurities contained therein.
 2. The systemaccording to claim 1, wherein the coating layer is made of a materialselected from the group consisting of SiC, SiO₂, and SiN.
 3. The systemaccording to claim 2, wherein the arm apparatus further comprises meansfor driving the main body in a vertical plane.
 4. The system accordingto claim 3, wherein a cassette for housing said one of the wafers islocated near the transfer position, and the arm apparatus transfers saidone of the wafers between the cassette and the boat.
 5. The systemaccording to claim 2, wherein the arm apparatus further comprises aplurality of stoppers arranged on the handling face of the main bodysuch that they engage a rim portion of the wafer to prevent the waferfrom shifting on the supporting protrusions while being transferred. 6.The system according to claim 5, wherein the stoppers compriseprojections arranged on the handling face of the main body and locatedhigher than the supporting protrusions.
 7. The system according to claim5, wherein the stoppers are arranged adjacent to the supportingprotrusions, respectively.
 8. The system according to claim 2, whereinthe supporting protrusions are fitted into and secured by recessesformed on the handling face of the main body.
 9. The system according toclaim 2, wherein the supporting protrusions are bonded to and secured onthe handling face of the main body.
 10. A heat processing system of thevertical type for processing wafers, comprising:(a) a process chamberfor housing a plurality of wafers at a time; (b) means for heating thewafers housed in the process chamber; (c) a wafer boat supporting thewafers in the process chamber with an interval between adjacent twowafers in a vertical direction; (d) boat conveying means for loading andunloading the boat into and out of the process chamber with the waferssupported on the boat; (e) an arm apparatus for transferring the wafersinto and out of the boat, which is set substantially in a verticalstate, at a transfer position outside the process chamber, the armapparatus comprising, an arm main body having a wafer handling face,driving means for driving the main body in a horizontal plane, aplurality of supporting protrusions arranged on the handling face of themain body and having wafer supporting top portions, respectively, tosupport one of the wafers, the supporting protrusions being formedseparately from and attached to the main body, a CVD coating layercoating the top portion of each of the supporting protrusions, thecoating layer being made of a material containing Si therein and havinga high hardness such that the coating layer does not contaminate thewafer with dust generated therefrom and impurities contained therein,and a plurality of stopper projections arranged on the handling face ofthe main body such that they engage a rim portion of the wafer toprevent the wafer from shifting on the supporting protrusions whilebeing transferred, wherein the stopper projections are arranged adjacentto the supporting protrusions, respectively, and extend higher than thesupporting protrusions.
 11. The system according to claim 10, whereinthe coating layer is made of a material selected from a group consistingof SiC, SiO₂, and SiN.
 12. The system according to claim 11, wherein thesupporting protrusions comprise at least four protrusions and thestopper protrusions comprise at least four protrusions.
 13. The systemaccording to claim 12, wherein the arm apparatus further comprises meansfor driving the main body in a vertical plane.
 14. The system accordingto claim 13, wherein a cassette for housing said one of the wafers islocated near the transfer position, and the arm apparatus transfers saidone of the wafers between the cassette and the boat.
 15. The systemaccording to claim 12, wherein the supporting protrusions are fittedinto and secured by recesses formed on the handling face of the mainbody.
 16. The system according to claim 12, wherein the supportingprotrusions are bonded to and secured on the handling face of the mainbody.